Reactive power management

ABSTRACT

Methods and systems for controlling a reactive power contribution to reactive power flowing in an electricity distribution network, so as to optimize this reactive power flow are described. A reactive power characteristic of electrical power flowing in the electricity distribution network is detected at a power device. The reactive power characteristic relates to a reactive power component of electricity flowing in the network. On the basis of the detected reactive power characteristic a reactive power contribution to the electricity distribution network is controlled so as to adjust a value of the detected reactive power characteristic. This enables individual power consumption and/or provision devices to react autonomously to local variations in the electricity distribution network, and to provide a reactive power contribution, to drive the detected reactive power characteristic towards a desired value.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 USC 119 of UK PatentApplication GB 1008685.8 filed May 25, 2010 and UK Patent Application GB1013987.1 filed Aug. 20, 2010, the entire disclosure of each of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to management of reactive power flow in anelectricity distribution network. In particular, but not exclusively, itrelates to devices and methods for controlling reactive power flow atpower consumption and/or provision devices.

BACKGROUND OF THE INVENTION

Supply of electricity from providers such as power stations, toconsumers, such as domestic households and businesses, typically takesplace via an electricity distribution network. FIG. 1 shows an exemplaryelectricity distribution network 1 comprising an electricitytransmission grid 100 and an electricity distribution grid 102. Thetransmission grid is connected to generating plants 104, which may benuclear plants or gas-fired plants, for example, from which it transmitslarge quantities of electrical energy at very high voltages (in the UK,for example, this is typically of the order of 204 kV; however thisvaries by country), using power lines such as overhead power lines, tothe distribution grid 102. The transmission grid 100 is linked to thedistribution grid 102 via a transformer node 106, which includes atransformer 106 which converts the electric supply to a lower voltage(in the UK, for example, this is typically of the order of 50 kV;however, this varies by country) for distribution in the distributiongrid 102. The distribution grid in turn links, via substations 108comprising further transformers for converting to still lower voltages,to local networks such as a city network 112 supplying domestic users114, and to industrial consumers such as a factory 110. Smaller powerproviders such as wind farms 116 may also be connected to thedistribution grid 102, and provide power thereto. Electric power istypically transmitted through all parts of the electricity distributionnetwork in the form of a sinusoidal alternating current (AC) wave.

Electric power consuming devices connected to the network at, forexample, the site of a domestic user 114, act as a load on the network,drawing power therefrom. The load provided by each such device istypically not purely resistive, but includes a reactive element, due tocapacitive and/or inductive elements in the devices. The reactivecomponent of the load may be particularly large in devices such aselectric motors and transformers, which have a high inductance, anddevices which have a high capacitance. Such devices produce a reactivecurrent component which flows at ±90° to the voltage; this results in aphase shift in the current flowing in the network with respect to thevoltage.

The term “reactive power” is used herein to refer to the product of areactive current component and the voltage flowing at a given locationof the network. This reactive power results in no net energy transfer tothe device, but it does have an effect on the network and on powersuppliers, as described below. The term “real power” is used herein torefer to the rate of electrical energy consumption by a power consumingdevice. The term “power factor” is used herein to refer to the ratio ofreal power to the vector sum of the real power and reactive power.

Whilst the effect of the reactive load of an individual device on thecurrent-voltage phase difference in the network may be small, thecumulative effect from multiple devices can be significant. The largerthe current-voltage phase difference becomes, the greater the size ofthe current that must be supplied to a device in order to supply a givenreal current (i.e. the current component flowing in phase with networkvoltage), and therefore a given amount of real power. Further, energylosses in the network, due to e.g. heating of electric power lines,depend on the total current flow, irrespective of whether the currentflow is real or reactive. Accordingly, such phase differenceseffectively increase the size of the total current that must begenerated and supplied by a power provider in order to meet the demandsof its customers; this places an economic burden on the power supplier,increasing the cost of electrical power generation. Similarly, theamount or resources consumed by the power provider in order to supply agiven amount of power to a consumer is increased, which may haveundesirable environmental consequences.

Further, network elements such as transformers and power lines aredimensioned according to total the size of the current (whether real orreactive) flowing in the network; their operation is therefore adverselyeffected by any reactive current flow (due to thermal losses and so on).

Conventionally, efforts to reduce the current-voltage phase differenceof power flowing in such networks have focussed on minimising thereactive power contribution of, and/or creating an appropriate amount ofcompensating reactive power at, large scale power suppliers, andcreating compensation for reactive power at transformer stations withinthe electricity distribution network. For example, a power station mayuse banks of capacitors and/or inductors, independently or underinstructions from the network operator, to adjust the reactive powercontribution of the power station. However, reactive power compensationis effective only at short distances (due to e.g. thermal losses), and,furthermore, the current-voltage phase difference may vary significantlyfrom location to location within the electricity distribution network;this means that reactive power compensation at a small number of largescale power providers does not effectively compensate for localisedcurrent-voltage phase differences.

Some large scale consumers of reactive power may also use some means ofminimising their own reactive power contribution to the network by usingsupplementary devices to compensate for the reactive power theygenerate, such as switched capacitors or an unloaded synchronous motor;indeed, some power providers encourage industrial consumers (such asfactories) to contribute less reactive power by charging for reactivepower contributions in addition to real power consumption. These methodsall focus on minimising the contribution of individual devices to acurrent-voltage phase difference in the network.

U.S. Pat. application US2009/0200994 describes a distributed system ofrenewable energy sources each including circuitry for generatingreactive power on demand. Each of the renewable energy sources is incommunication with a central controlling “Network Operations Centre”,which remotely controls reactive power production by renewable energysources. The Network Operations Centre receives a request from a utilitycompany (i.e. a power provider) for a required amount of reactive power;in response the Network Operations Centre calculates an optimum reactivepower contribution required from each of the renewable energy sourcesunder its control to produce the necessary compensation, and sendscommands to the renewable energy sources accordingly. This provides amethod of actively compensating for current-voltage phase differencesthat may be present in the network. However, the system of US20090200994requires central control and cannot react to more localised changes inlocal network voltage-current phase differences.

It is an object of the present invention to at least mitigate some ofthe problems of the prior art.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, there is provided areactive power control device for use with a power device, the powerdevice being for consuming power from, or providing power to, anelectricity distribution network when connected thereto, the reactivepower control device comprising:

detecting means for detecting, at the power device, a reactive powercharacteristic of electrical power flowing in the electric powerdistribution network, the reactive power characteristic relating to areactive power component of electricity flowing in the electricitydistribution network; and

control means arranged, to control, on the basis of the detectedreactive power characteristic, a reactive power contribution to theelectricity distribution network by the power device, so as to adjust avalue of the detected reactive power characteristic.

By controlling provision of reactive power at a power device on thebasis of a reactive power characteristic measurement made at the powerdevice, real-time compensation can be provided for local variations inreactive power. The reactive power control device enables compensationfor these local variations through autonomous variation of a reactivepower contribution based on local measurement, and do not requirecentral control.

In preferred embodiments, the reactive power control device is arrangedto determine a value of the detected reactive power characteristic andcompare the determined value with a predefined value, and the controlmeans is arranged to control the reactive power contribution so as toreduce a difference between the detected value and the predeterminedvalue.

Thus, the reactive power control device can be arranged to control apower device to produce a reactive power contribution which drives thereactive power of electricity in the electricity distribution networktowards a predetermined optimum value.

The reactive power characteristic may relate to a phase differencebetween voltage and current of electricity flowing in the electricitydistribution network; the predefined value may comprise a predefinedphase difference value.

Preferably, the reactive power compensation device is arranged todetermine, based on the detected reactive power characteristic, whetherthe reactive power component is more inductive than the predefined valueor whether it is more capacitive than the predefined value, and thecontrol means is arranged to:

control the power device to contribute capacitive reactive power to theelectricity distribution network in response to a determination that thedetected reactive power component is more inductive than the predefinedvalue; and

control the power device to contribute inductive reactive power to theelectricity distribution network in response to a determination that thedetected reactive power component is more capacitive than the predefinedvalue.

Thus, the reactive power compensation device may control the powerdevice to compensate for either inductive or capacitive power flows inthe electricity distribution network.

In some embodiments, the detecting means is arranged to detect a changein the reactive power characteristic, and the control means is arrangedto change the reactive power contribution to the electricitydistribution network, in response to the detecting means detecting atransition of a value of the reactive power characteristic across athreshold value.

Preferably, the control means is arranged to:

change a value of the reactive power contributed to the electricitydistribution network by the power device from a first predeterminedvalue to a second predetermined value in response to a value of thedetected reactive power component changing from a value less than afirst threshold value to a value greater than the first threshold value;and

change the reactive power contributed to the electricity distributionnetwork by the power device from the second predetermined value to thefirst predetermined value in response to a value of the reactive powercomponent changing from a value greater than a second threshold value toa value less than the second threshold value, the second threshold valuehaving a magnitude less than the first threshold value.

This provides a hysteresis in the provision of reactive power to theelectricity distribution network, preventing oscillations occurring inthe functioning of the device, and disrupting current flow in theelectricity distribution network.

Preferably, the control means is arranged to change the reactive powerin response to a predetermined time period elapsing after the detectingmeans detects the change in the reactive power characteristic. This alsoprevents oscillations from being introduced into the electricity flow inthe system.

In some embodiments, the electricity flowing in the electricitydistribution network comprises an alternating current electricity flowhaving a predetermined period, and the control means is arranged tocontrol a switching means to selectively interrupt provision of power tothe power device during one or more portions of the predeterminedperiod. In some embodiments, the control means comprises a currentmodulation means, such as pulse width modulation means, and the powercharacteristic comprises a duty cycle characteristic of the power of thepower consumption device.

In some embodiments, the power device comprises a power provision devicefor providing electric power to the electricity distribution network,the power provision device is arranged to provide a direct current, andthe control means is arranged to control a direct current to alternatingcurrent conversion means.

The reactive power control device may comprise means for detecting oneor more electricity quality characteristic of electricity flowing in theelectricity distribution network, and the control means is arranged tocontrol a characteristic of power provided and/or consumed by the powerdevice so as to alter said detected electricity quality characteristic,wherein the detected one or more electricity quality characteristiccomprising at least one of; harmonics in the electricity flow; random orrepetitive variations in voltage; network imbalance; oscillations inpower flow; transients in the power flow. Thus, embodiments of thepresent invention may be used to correct anomalies in an electricitydistribution network other than reactive power characteristics.

The power provision device for providing electric power to theelectricity distribution network, the power provision device maycomprise at least one of: a photovoltaic generation device, a personalelectric vehicle, a personal electric bicycle and a domestic renewableenergy source such as a CHP power generation device. The power devicemay comprise a power provision device arranged to provide a power to theelectricity distribution network of typically up to approximately 10 kW,as the state of the art allows.

In some embodiments, the electricity distribution network comprises atransmission grid and a distribution grid, the transmission grid beingconnected to the distribution grid via one or more transformers andproviding electric power thereto, and the distribution grid providingpower to a plurality of domestic and/or industrial users, wherein thepower device is for consuming power from, or providing power to, saiddistribution grid. Thus, embodiments of the present invention may beused in a transmission grid, for example in a local or city network.This enables a corrective contribution to be made to local reactivepower flows in such networks.

In some embodiments, the reactive power control device comprises acommunications interface for receiving an activation signal from acontrol centre, and the control means is arranged to perform the controlof reactive power contribution in response to receipt of the activationsignal at the communications interface. This enables a central entity toactivate and/or deactivate of one or more reactive power controldevices; this may be useful in applications of the present inventionwhere groups of reactive power devices are used to provide reactivepower compensation on an on-demand basis, for example as part of ascheme in which reactive power is sold to an electric power provider.

In accordance with a second aspect of the present invention, there isprovided a system for use in controlling reactive power flow in anelectricity distribution network, the system comprising a distributedplurality of reactive power control devices as described above, eachcontrolling a respective power device connected to the electricitydistribution network. Embodiments of the present invention may beimplemented with distributed groups of power supply devices arranged tocollectively provide compensatory reactive power compensation to theelectricity distribution network.

The system may also comprise a control centre for sending the activationsignal to each of the distributed plurality of reactive power controldevices.

In preferred embodiments, different ones of the plurality of reactivepower control devices are arranged to initiate performance of thecontrol at different intervals after receiving the activation signal.This prevents all of the plurality of devices from activatingsimultaneously, causing a sudden change in electricity power flow in theelectricity distribution network.

In some embodiments, the reactive power control devices comprises meansfor monitoring one or more performance characteristics of a respectivepower device, and for each device being arranged to transmit dataindicative of the one or more performance characteristic to the controlcentre, the one or more performance characteristic comprising at leastone of: a value of a detected reactive power component flowing in theelectricity distribution network, an amount of reactive power providedto the electricity distribution network by a respective power device andtimes of performance of the control of provision of reactive power.

Additionally, or alternatively, the reactive power control devices maycomprise means for measuring an electricity quality characteristic ofelectricity flowing in the electricity distribution network, and each ofthe reactive power control devices is arranged to transmit dataindicative of the one or more electricity quality characteristic, theone or more electricity quality characteristic comprising at least oneof: a reactive power characteristic harmonics in the electricity flow;random or repetitive variations in voltage; network imbalance;oscillations in power flow; and transients in the power flow.

This enables the control centre to obtain data relating to measurementsmade at various distributed points throughout the electricitydistribution network; such data may be valuable for a power provider inmonitoring network conditions, for example.

In accordance with a third aspect of the present invention, there isprovided a method for use in controlling reactive power flow in anelectricity distribution network connected to one or more power devices,the one or more power devices being arranged to consume power from,and/or provide power to, the electricity distribution network, themethod comprising:

detecting, at the power device, a reactive power characteristic ofelectrical power flowing in the electric power distribution network, thereactive power characteristic relating to a reactive power component ofelectricity flowing in the electricity distribution network; and

controlling, on the basis of the detected reactive power characteristic,a contribution of reactive power to the electricity distribution networkby the power device, so as to adjust a value of the detected reactivepower characteristic.

Further features and advantages of the invention will become apparentfrom the following description of preferred embodiments of theinvention, given by way of example only, which is made with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art electricity distribution network;

FIG. 2 shows a reactive power control device, a power consumption and/orprovision device, an electricity distribution network and connectionstherebetween, in accordance with an embodiment of the present invention;

FIG. 3 a shows power consumption cycles of a power consumption device;

FIG. 3 b shows a first type of controlled power consumption of power bya power consumption device in accordance with an embodiment of thepresent invention;

FIG. 3 c shows a second type of controlled power consumption of power bya power consumption device in accordance with an embodiment of thepresent invention;

FIG. 3 d shows a third type of controlled power consumption of power bya power consumption device in accordance with an embodiment of thepresent invention;

FIG. 4 shows a reactive power control device, a power provision device,an electricity distribution network and connections therebetween, inaccordance with an embodiment of the present invention;

FIG. 5 a is a graph showing voltage versus time at the output of a DCpower provision device;

FIG. 5 b is a graph showing the variation of voltage with time atterminals of an H-bridge controlled by a reactive power control deviceoperating in a first mode in accordance with an embodiment of thepresent invention;

FIG. 5 c is a graph showing the variation of voltage with time atterminals of an H-bridge controlled by a reactive power control deviceoperating in a second mode in accordance with an embodiment of thepresent invention;

FIG. 5 d is a graph showing the variation of voltage with time atterminals of an H-bridge controlled by a reactive power control deviceoperating in a third mode in accordance with an embodiment of thepresent invention;

FIG. 5 e is a graph showing the variation of current with time ofcurrent provided to an electricity distribution network by a powerprovision device controlled by a reactive power control device operatingin the first mode in accordance with an embodiment of the presentinvention;

FIG. 5 f is a graph showing the variation of current with time ofcurrent provided to an electricity distribution network by a powerprovision device controlled by a reactive power control device operatingin the second mode in accordance with an embodiment of the presentinvention;

FIG. 5 g is a graph showing the variation of current with time ofcurrent provided to an electricity distribution network by a powerprovision device controlled by a reactive power control device operatingin the second mode in accordance with an embodiment of the presentinvention;

FIG. 6 is a flow diagram showing steps performed by a reactive powercontrol device when controlling a power consumption and/or provisiondevice in accordance with an embodiment of the present invention; and

FIG. 7 is a graph showing measured network reactive power versusgenerated reactive power at a power consumption and/or provision devicein accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows a reactive power control device 200 for use in controllinga phase difference between the current and the voltage of electricityflowing in an electricity distribution network 1 at the location of apower device 202. The power device 202 may be a consumption device, forexample, a low power consumption device, having a power rating of lessthan 500 W, such as an energy saving lamp, a mobile telephone charger,computing device supply, a medium sized power device, having a powerrating of between 500 W and 10 kW, such as a personal electric vehicle(PEV), or a large power device, having a power rating of more than 10kW, such as industrial machinery located at a factory. It should benoted that the devices may be single-phase or multi-phase; in the lattercase the above power ratings apply per phase.

When the power device 202 comprises a power consumption device power issupplied to the power device 202 from the electricity distributionnetwork 1, typically in the form of an alternating current, such as asinusoidal alternating current.

The power device 202 may alternatively or additionally comprise a powerprovision device that provides power to the electricity distributionnetwork 1. The power provision devices may comprise a power generationdevice that generates power using, for example, photovoltaic cells, orit may comprise a device that simply stores energy and releases it whenrequired. Some devices may function both as a power consumption deviceand a power provision device; for example, personal electric vehicles(PEV) typically have the capacity to store a large amount ofelectricity. This means that, in addition to being consumers of power,they can be used as a source of power for the network at times of highdemand, with electricity stored in the battery of the PEV being fed backto the network at such times.

In embodiments of the present invention, the power device 200 istypically connected to the distribution grid 102 part of the electricitydistribution network 1 described above with reference to FIG. 1, forexample to a local network such as a city network.

The reactive power control device 200 may be implemented as an integralpart of the power device 202 or a peripheral device thereof; forexample, it may be implemented as part of an AC/DC converter for amobile telephone charger or PEV. In other arrangements the reactivepower control device may be implemented as a stand-alone device, whichcan be connected to the electricity distribution network 202, and towhich power devices 202 can be interchangeably connected so that thepower device receives power from, and/or provides power to, theelectricity distribution network 202 via the reactive power controldevice 200.

The exemplary reactive power control device 200 shown in FIG. 2comprises a reactive power meter 203, a control unit 204, and a currentcontrol device 212. The reactive power meter 203 detects and measures areactive power characteristic (RPC) relating to a reactive powercomponent of power flowing in the electricity distribution network 1 atthe location of the power device 202. The reactive power meter 203 maycomprise a phase detector which is commonly found in phase locked loop(PLL) circuits, a voltmeter and ammeter combination, and a clock, whichmeasure voltage and current of electricity in the electricitydistribution network 1 at the location of the power device 202, andtiming characteristics thereof, or any other device capable of measuringa reactive power characteristic of the electricity flowing in theelectricity distribution network. The reactive power meter 203 sendsmeasurement signals to the control unit 204 indicative of RPCs itdetects. The measurement signals may include data indicative of themagnitude and/or sign (i.e. whether the current lags or leads thevoltage) of a reactive power component.

Many of the following examples are described with reference to adetected phase difference; however, in some embodiments a different RPCmay be detected and used to determine a mode of the reactive powercontrol device 200, for example, a magnitude of the reactive powercomponent, or a power factor of the electricity flow. Further, where itis described herein that the reactive power meter 203 “measures” or“detects” a RPC, this should be understood to include cases where thereactive power meter 203 provides data to the control unit 204 fromwhich the latter may derive the RPC; for example, the reactive powermeter 203 may provide the control unit with data indicative of timingsof peak voltage and peak current of alternative power flowing theelectricity distribution network 1, from which the control unit 204 mayderive a phase difference.

The control unit 204 comprises a processor 206, a data store 210 and acommunications interface 208. The processor 206, which may beimplemented as, for example, a programmable device or using simple logiccircuitry, receives the measurement signals sent by the reactive powermeter 203, and, based on these signals, determines a current-voltagephase shift of electricity flowing in the network and sends controlsignals to the current control device 212. The data store 210 may beused to record data such as data indicative of measurements made by thereactive power meter 203, control signals sent by the processor 206 tothe current control device 212, and so on. The I/O interface 208 may beused to communicate with a control centre to provide it with datarecorded in the data store and/or receive activation and/or deactivationsignals from the control centre; the control centre and its interactionswith the reactive power control devices will be described in more detailbelow.

The current control device 212 receives control signals from theprocessor 206 and, based on these signals, controls current flow to andor from the power device 202 such that the power device 202 contributesa reactive power flow to the electricity distribution network 1, thereactive power flow contribution from the power device 202 beingselected as either a capacitive or an inductive power flow so as todrive the current-voltage phase difference towards a set optimum value,as discussed below.

Exemplary operations of the current control device 212 are now describedwith reference to FIGS. 3 a to 3 d, which show a variation of voltagewith time at the power device 202. For clarity, the power device 202 inthese examples is assumed to be a power consumption device and isreferred to as such; however, it will be understood that the examplesprovided apply equally, mutatis mutandis, to power provision devices.

The shaded areas of FIGS. 3 a to 3 d represent periods of time when thecurrent control device 212 controls the power consumption device 202 todraw current from the electricity distribution network 1; the unshadedareas represent periods of time when the current control device 212controls the power consumption device 202 to draw no current from theelectricity distribution network 1. This control may be performed byoperating a switching device, typically a semiconductor switchingdevice, to selectively connect and disconnect the power consumptiondevice 202 to and from the electricity distribution network 1. In thisway, the reactive power control device 200 can vary the duty cycle ofthe power consumption device 202 such that the power consumption 202device consumes power asymmetrically over a given cycle, therebyadjusting the amount of reactive power it provides to the electricitydistribution network 1, as is now described in more detail. Bymodulating the current provided to the power consumption device, thereactive power contribution of the power consumption device 202 can bevaried without using any additional means of generating reactive power,such as switched capacitors. Further, the current modulation methodenables a faster response to changes in network conditions than ispossible with these additional means.

In a first mode of operation of the reactive power control device 200shown in FIG. 3 a), the power consumption device 202 draws current fromthe electricity distribution network 1 over the whole of the voltagecycle. In this mode, the current control device 212 does not exert anyinfluence on the power consumption device 202 i.e. the switch referredto above is in an “on” position for the whole of the voltage cycle. Inthis mode of operation, the current control device 212 has no effect onthe current-voltage phase difference in the network. This mode ofoperation is referred to herein as a “neutral mode”.

In a second mode of operation shown in FIG. 3 b), the reactive powercontrol device 200 controls the power consumption device 202 to drawcurrent only for a time period T1 after each time the voltage at thedevice 202 crosses zero. This results in a current component whichvaries according to the same frequency as the voltage, but which leadsit i.e. the reactive power control device 212 controls the powerconsumption device 202 to provide a capacitive reactive power to theelectricity distribution network; conventionally, the phase differenceis assigned a positive value where the current leads the voltage.Following this convention, in this mode of operation, the reactive powercontrol device 200 controls the power consumption device 202 to generatea positive contribution to the current-voltage phase difference ofelectricity flowing in the electricity distribution network 1. Modes ofoperation in which the power consumption device 202 provides a positivecontribution to the current-voltage phase difference are collectivelyreferred to herein as a “capacitive mode”.

In a third mode of operation shown in FIG. 3 c), the current controldevice 212 controls the power consumption device 202 to draw currentonly for a time period T2 before each time the voltage crosses zero; thelength of time period T2 may be the same as or different to that of timeperiod T1. This results in a current component which varies according tothe same frequency as the voltage, but which lags it i.e. the currentcontrol device 212 controls the power consumption device 202 to providean inductive contribution to power flow in the network. Following theabove-described convention, in this mode of operation, the reactivepower control device 200 controls the power consumption of the powerconsumption device 202 to produce a negative contribution to thecurrent-voltage phase difference of electricity flowing in theelectricity distribution network 1. Modes of operation in which thepower consumption device 202 provides a negative contribution to thecurrent-voltage phase difference are collectively referred to herein asan “inductive mode”.

Although FIGS. 3 a) to 3 c) show three modes of operation of the currentcontrol device 212, in some embodiments of the present invention, thereactive power control device 200 has a different number of modes. Forexample, it may have different modes in which the values of T1 and/or T2are varied so as to produce a greater or smaller reactive contributionfrom the power consumption device 202. It may additionally oralternatively have one or more modes of operation in which the voltagecycle is divided into many time slots, with current being drawn fromselected ones of the slots in order to adjust the reactive contributionof the power consumption device 202. An example of such a mode ofoperation is shown in FIG. 3 d), in which each half cycle C is dividedinto 16 time slots, with power being drawn from selected ones of thetime slots; although only 16 slots are shown, for ease of understanding,typically each half-cycle C will be divided into many hundreds orthousands of slots, which enables a smoother distribution of powerconsumption over the voltage cycle. In the examples shown in FIG. 3 d),power is supplied to the power consumption device during every othertime slot in the early part of each half-cycle C, with power beingsupplied only every third time slot during the later part of each halfcycle C, resulting in a capacitive reactive power contribution to theelectricity distribution network.

In some embodiments of the present invention, the current control device212 comprises a Pulse-Width Modulation (PWM) unit, and operatesaccording to a PWM method, as is now described. In a PWM method, eachcycle of the power supply from the electricity distribution network 1 isagain divided into slots, for example a few tens of slots, with theproportion of time during each slot that power is supplied to the powerconsumption device 212 being varied according to the slots position inthe cycle; for example, the power consumption device 202 could be run at45% of capacity during the first and third quarters of each cycle, andat 55% during the second and fourth quarters to produce a “lagging”inductive power contribution. This may be implemented by dividing eachof the time slots into sub-slots and supplying power to the powerconsumption device only during a proportion of the sub-slots of a givenslot, for example.

As mentioned above, although the above examples described in relation toFIGS. 2 and 3 refer to the power device 202 as a power consumptiondevice, in some embodiments of the present invention the power device202 is a power provision device which provides power to the electricitydistribution network 1, with provision of power rather than theconsumption thereof being controlled by the reactive power controldevice 200. In the latter case, an alternating current provided by thepower device 202 is modified in accordance with the above-describedprinciples. This method is particularly suitable when the currentprovided by power device 202 is in AC form, or is converted to an ACform before being processed as described above.

However, in cases where the power device 202 provides a DC current, forexample if the power device is a solar panel or electricity storagedevice, such as a PEV or personal electric bicycle battery, it may beconvenient to modulate the current as part of a DC to AC conversionprocess. Further, in cases where AC power provided by the power device202 is variable or of low quality, for example in a domestic biofuelgenerator, it may be convenient to convert the AC power to a stable DCpower before re-converting it to an AC power according to a method as isnow described. The AC/DC conversion may be implemented using a temporaryor intermittent energy store such as a battery.

FIG. 4 shows details of a reactive power control device 200 that may beused as part of a (DC/AC) conversion device (often referred to as an“inverter”), as is described below. In this example, the reactive powercontrol device 200 is used with a DC power provision device 202 a, andthe current control device 212 comprises an H-bridge 412, an inductor404 and a transformer 406, functions of which are described below.

The DC supply from a DC power provision device 202 a is connected to anH-bridge 412, which comprises four switches 400 a to 400 d, which aretypically implemented as transistor or other semiconductor switches. TheH-bridge 412 is controlled by the reactive power control unit 204 tocontrol a reactive power contribution of the DC power provision device202 a. The H-bridge is connected via terminals 402 a and 402 b to aninductor 404 and subsequent components, which are described below.

By altering the configuration of the switches 400 of the H-bridge 412,it is possible to alter the polarity of the terminals 402 a and 402 b ofthe H-bridge 412. In a configuration in which diagonally opposingswitches 400 a and 400 d are open and switches 400 b and 400 c areclosed, H-bridge terminal 402 a is electrically negative (i.e. currentflows through terminal 402 a towards the negative terminal 401 a of thepower provision device 202 b), whilst H-bridge terminal 402 b iselectrically positive (i.e. current flows through terminal 402 b of theH-bridge 412 away from the positive terminal 401 a of the powerprovision device 410 b). Conversely, in a configuration in whichswitches 400 a and 400 d are open and switches 400 b and 400 c areclosed, H-bridge terminal 402 a is electrically positive and H-bridgeterminal 402 b is electrically negative.

The processor 206 of the control unit 204 controls the switchingconfigurations of the H-bridge so as to produce an AC signal fromterminals 402 a and 402 b. The signal from the H-bridge 412 is fed to aninductor 404, which smoothes the AC signal, and from there to atransformer 406, which adjusts the signal to have voltage suitable fortransmission in the electricity distribution network 1, to which thesignal from the transformer 406 is fed.

FIG. 5 a is a graph of potential difference (V) between output terminals401 a and 401 b of the power provision device 202 b, against time (T).The potential difference is shown as non-varying; in practice, there maybe some variation with time, according to, for example, weatherconditions, if the power provision device 202 a is a solar powergenerator.

FIGS. 5 b to 5 d are graphs showing a potential difference betweenH-bridge terminals 402 a and 402 b against time, when the H-bridge 412is being controlled in different modes according to embodiments of thepresent invention. FIGS. 5 e to 5 g show a corresponding variation ofcurrent (I) with time at the output terminals 408 a and 408 b of thetransformer 406, i.e. the current supplied to the electricitydistribution network 1. In each of the examples described with referenceto FIGS. 5 b to 5 d, the control unit 204 controls the H-bridge 412 suchthat the switches 400 a to 400 d operate in a periodically repeatingon-off sequence to produce an alternating current at the outputterminals 408 a and 408 b of the transformer. The alternating currentproduced is arranged to be in phase with, and have a half-cycle C′length corresponding to that of the electricity flowing in theelectricity distribution network 1.

FIG. 5 b shows the variation of voltage against time at the H-bridgeterminals 402 a and 402 b when the control unit 204 is controlling theH-bridge 412 according to a neutral mode of operation. In this mode ofoperation, the control unit 204 controls the switches 400 of theFI-bridge 412 in a repeating on-off sequence, which is symmetric aboutthe centre point of each half-cycle C′, such that the current suppliedto the electricity distribution network 1 is a symmetrical sinusoidalalternating current, as shown in FIG. 5 e. Since the on-off pattern, andthe current provided to the electricity distribution network issymmetric about the centre point of each half cycle C′, no reactivepower contribution is made to the electricity flowing in the electricitydistribution network.

FIG. 5 c shows the variation of voltage against time at the H-bridgeterminals 402 a and 402 b when the control unit 204 is controlling theH-bridge according to a capacitive mode of operation. In this mode ofoperation, the control unit 204 controls the switches 400 of theH-bridge 412 in a repeating on-off sequence which is asymmetric aboutthe centre point of each half-cycle, such that the current supplied tothe electricity distribution network 1 is an asymmetric alternatingcurrent, as shown in FIG. 5 f. The proportion of time during whichcurrent flows through the H-bridge 412 is higher during the first halfof each half-cycle C′, such that the current supplied to the electricitydistribution network leads the voltage of electricity flowing therein;in this mode, a capacitive contribution is therefore made to theelectricity flowing in the electricity distribution network 1.

FIG. 5 d shows the variation of voltage against time at the H-bridgeterminals 402 a and 402 b when the control unit 204 is controlling theH-bridge according to an inductive mode of operation. In this mode ofoperation, the control unit 204 controls the switches 400 of theH-bridge 412 in a repeating on-off sequence which is asymmetric aboutthe centre point of each half-cycle, such that the current supplied tothe electricity distribution network 1 is an asymmetric alternatingcurrent, as shown in FIG. 5 g. The proportion of time during whichcurrent flows through the H-bridge 412 is higher during the second halfof each half-cycle C′, such that the current supplied to the electricitydistribution network lags the voltage of electricity flowing therein; inthis mode, an inductive contribution is therefore made to theelectricity flowing in the electricity distribution network 1.

The embodiments described with reference to FIG. 4 and FIGS. 5 a to 5 gprovide a further method of modulating current flow from a powerprovision device 202 to vary the reactive power contribution of thepower provision device 202 can be varied without using any additionalmeans of generating reactive power, such as switched capacitors. In theembodiments described, a reactive power contribution is controlled byproducing an AC signal which is asymmetric about the centre point ofeach half-cycle C′, but which crosses the zero point at the samefrequency at, and at the same timing of, the voltage signal flowing inthe electricity distribution network 1. Additionally or alternatively, areactive power contribution may be produced by shifting the timing atwhich the signal crosses the zero point, so as to be out of phase withthe voltage signal flowing in the electricity distribution network 1.

In the above examples, the reactive power control device operates in afinite number of discrete modes, in which the 212 provides reactivepower of a predefined magnitude in each of the modes. In some cases,there are regulations prescribing a maximum allowable power factor thatmay be provided by a power consumption device 202. For example, EuropeanCommunity regulations prescribe that devices having a power rating of upto 25 W must have a power factor of 0.5 or more, and that devices havinga power rating of 75 W or greater must have a power factor of 0.9 ormore (see IEC/EN 61000-3-2). Accordingly, it may be advantageous for thecurrent control device 212 to be arranged to provide the maximumallowable output in all cases that the measured current control device212 is active. However, in some embodiments, the current control device212 may be arranged to vary the magnitude of the reactive power providedcontinuously in accordance with the measured value of thecurrent-voltage phase difference.

The number and type of modes of operation may be chosen according tocharacteristics, such as the power rating, of the power device 202and/or regulations prescribing a maximum allowed reactive powercontribution. Further, the current control device 212 is not limited tothe examples described; for example, a device, which could include avariable resistance, could be used which provides some proportion of thetotal available current at any given point in the current cycle, inconjunction with or as an alternative to the switching devices describedabove.

By controlling the power consumption of the power consumption device 202according to the modes of operation of the reactive power control device200 described above, a capacitive reactive power contribution and/or aninductive reactive power contribution to the power flow in theelectricity distribution network 1 can be provided which is independentof any inductive and/or capacitive reactance that the components of thepower device 202 may themselves possess. Thus, although the power unit202 may make, for example, an inductive contribution to the power flow,due to inductive components therein, such as magnet coils etc., thecontribution to the power flow due to the operation of the reactivepower control device 200 may be, for example, capacitive.

FIG. 6 is a flow diagram showing exemplary steps performed by thereactive power control device 204 in controlling the power device 202 inaccordance with some embodiments of the present invention. At step S600the reactive power meter 203 measures a current-voltage phase differenceof electricity flowing in the electricity distribution network at thelocation of the power device 202.

At step S602, the processor 206 in the control unit 202 determines,based on the measurement made at step S600 whether the phase differenceis within a predetermined range, which may be a predetermined range ofthe phase differences, within which the phase difference does notrequire adjustment. If the processor 206 determines that the phasedifference is within the predetermined range, no adjustment to thereactive contribution of the power device 202 is required, so theprocess proceeds to step S604 in which the processor 206 selects aneutral mode of the reactive power control device 200, and sends acommand signal to the current control device 212 to operate in theselected neutral mode.

If, on the other hand, the processor determines at step S602 that thephase difference is not within the predetermined range, the processproceeds to step S606 where the processor 206 determines whetherinductive compensation is required, for example by determining whetherthe phase difference measured at step S600 is outside of thepredetermined range on the inductive side. If it determines that aninductive contribution is required, the process proceeds to step S608,in which the processor selects a capacitive mode, so that the currentcontrol device 212 controls the power device 202 to provide an inductivecontribution to power flow in the electricity distribution network 1,and sends a command signal to the current control device 212 to operatein a capacitive mode.

If it is determined at step S606 that inductive compensation to thephase difference is not required, it is concluded that a capacitivecontribution is required, and the process proceeds to step S610 in whichthe processor 206 selects an inductive mode, so that the current controldevice 212 controls the power device 202 to provide an inductivecontribution to power flow in the electricity distribution network, andsends a command signal to the current control device to operate in aninductive mode.

In this way the reactive power control device 200 controls the flow ofcurrent, and thereby power, to and/or from the power device 202 inresponse to a locally detected current-voltage phase difference ofelectricity flowing in the electricity distribution network 1. Thiscontrol of current flow produces a reactive power contribution whichadjusts the phase difference of the electricity flowing in theelectricity distribution network. In cases where the power consumptionand/or provision of the power device 202 is large, the correctivereactive power contribution from an individual device may provide asignificant adjustment to the phase difference of electricity flowing inthe electricity distribution network. Further, even where the powerconsumption and/or provision of individual power devices 202 is small,for example if the device is a low power device as described above, thecombined contribution from a distributed group of such devices can havea significant effect on current-voltage phase difference of electricityflowing in the electricity distribution network, as is described in moredetail below.

Typically, the conditions of the network will vary with time;accordingly, in some embodiments of the present invention, the processdescribed above with reference to FIG. 6 is repeated continuously, or atintervals, with changes in the phase difference detected by the reactivepower meter resulting in different modes, and thereby different reactivepower contributions being selected.

In the above example, it was described that the reactive powercompensation device 200 determines whether the phase difference iswithin a predetermined range and selects a mode of operation independence on this determination; however, in some embodiments, there isno predetermined range. In these embodiments, the reactive powercompensation device 200 omits step S602, and proceeds directly to stepS606. In other words, in these embodiments, there is no “neutral mode”of operation of the reactive power compensation device, and the reactivepower compensation device 200 switches between capacitive and inductivemodes depending on whether the reactive power flow is determined to bemore inductive or more capacitive than a predefined value. In some casesit is desirable to provide a reactive power component having a signwhich always opposes the sign of the detected reactive power component(i.e. to provide a capacitive component if the detected reactive powercomponent is inductive, and vice versa), so that the predefined valuementioned above is zero; in other cases, the optimum current-voltagephase difference may be a non-zero value. This is because thecurrent-voltage phase difference may vary as it travels through theelectricity distribution network 1, due to reactive contributions fromtransformers and other components in the electricity distributionnetwork 1. The optimum situation from the perspective of efficiency ofpower transmission is therefore not necessarily for the phase differenceto be zero at the location of the power device 202; it may instead befor the phase difference to, for example, slightly capacitive (e.g. inthe range 2 to 5 degrees) at the power device 202, so that, if the phasedifference is slightly inductive at a power station, it will be zero atsome point in the middle of the network. Accordingly, the reactive powercompensation device 200 may be arranged to determine a value of thephase difference, compare this with a predefined optimum value, andprovide a reactive power contribution aimed at reducing the differencebetween the detected phase difference and the predefined value.

Since the predetermined range mentioned above is typically centred on(or at least includes) the optimum value, in some cases, especiallywhere the optimum value is set at or near to zero and/or where thepredetermined range mentioned above is relatively large, the end pointsof the range may be of opposite sign i.e. one end may correspond to acapacitive value, with the other end corresponding to an inductivevalue. In these cases, the step of determining whether inductivecompensation described above (step S606) may comprise simply determiningwhether the current-voltage phase difference measured at step S600 isinductive or whether it is reactive. In other cases, where the optimumvalue is set at a non-zero value, and especially where the predeterminedrange is relatively narrow, the predetermined range will comprise onlycapacitive values or only inductive values. In these cases, step S606may comprise comparing the phase difference measured with at step S600with the end points of the predetermined range to determine whether themeasured phase difference lies on the inductive side or the capacitiveside of the range.

In some cases there may be small fluctuations in the phase shift ofelectricity in the electricity distribution network 1; if thesefluctuations occur at or near the threshold values defining the endpoints of the acceptable range described above, they can cause thereactive power control device 200 to oscillate between modes; this mayexacerbate the oscillations, and produce instability in the currentflowing in the electricity distribution network 1. In order to reduce oreliminate these oscillations, a hysteresis may be used in relation tothe threshold values at which the reactive power control device 200switches between modes, as is now explained with reference to FIG. 5.

FIG. 7 is a graph showing the measured network phase difference versusthe generated reactive power of the power device 202. In the exampleshown, the power device 202 generates a reactive power of +P whenoperating in mode 2, zero reactive power when operating in mode 1, and areactive power of −P when operating in mode 3. Although the magnitude ofthe reactive power generated in this example is the same in both mode 2and mode 3, in some embodiments the magnitude may be different in eachof the modes.

FIG. 7 shows four threshold values, R₁ to R₄ at which the reactive powercontrol device 200 switches between modes, with R₁ and R₂ representingnegative (inductive) phase differences (|R₁|>|R₂|) and R₃ and R₄representing positive (capacitive) phase differences (|R₄|>|R₃|). As themagnitude of an inductive network phase difference increases, thereactive power control device switches from a neutral mode to capacitivemode once the value of the phase difference crosses R₁. However, in theopposite direction, as the magnitude of an inductive network phasedifference decreases, the reactive power control device 200 does notswitch from a capacitive mode to a neutral mode at R₁; instead itswitches at R₂. Similarly, for an increasing capacitive network phasedifference, the reactive power control device 200 switches from aneutral mode to an inductive mode, but for a decreasing capacitivenetwork phase difference, it switches at R₄. Thus, even if the phasedifference does fluctuate around one of the threshold values R₁ to R₄,there is no oscillation in the mode of operation of the reactive powercontrol device 200, because the latter is stable in a neutral modearound R₂ and R₃, stable in a capacitive mode around R₁ and stable in aninductive mode around R₄.

The example described above in relation to FIG. 7 refers to R₁ and R₂ asrepresenting inductive values and R₃ and R₄ as representing capacitivevalues; however, it will be understood that where the optimum valuementioned above is non-zero, all of R₁ to R₄ may be capacitive or theymay all be inductive. In this case, the reactive power control device200 may switch to a capacitive mode to provide a capacitive contributionwhen the detected phase difference crosses R₁, even if the detectedphase difference remains capacitive; similarly, the reactive powercontrol device 200 may switch to an inductive mode to provide aninductive contribution when the detected phase difference crosses R₄even if the detected phase difference remains inductive.

Additionally, or alternatively, to the hysteresis feature described, thereactive power control device may be arranged to delay the switchbetween modes when crossing a threshold value for some predeterminedtime period T3. This also prevents high frequency oscillations frombeing introduced into the system.

Furthermore, as mentioned above, in some embodiments of the presentinvention, a plurality of reactive power control devices 200 may bedistributed at different locations in the electricity distributionnetwork, each of the reactive power control devices 200 controlling arespective power device 202, thereby forming a system which can be usedto control reactive power flow in the electricity distribution network1. This may be particularly advantageous when used with low or mediumsized power devices 202, enabling a more significant adjustment toreactive power flow in the electricity distribution network than can beachieved by using individual devices. For example, it is estimated thataround 5% of all power consumption is due to lighting devices atcommercial and domestic premises. If all such lighting devices used areactive power control device 200 according to the present invention,and assuming that the methods described herein allow 5% of total powerusage of a power consumption device to be contributed as reactive power(a figure which is easily achievable using the methods describedherein), 0.25% of total network power capacity could be used to providemore efficient reactive power characteristics

In embodiments using distributed groups of reactive power controldevices 202 (though not limited to these embodiments), one or more ofthe values of the threshold values R₁ to R₄ at which the reactive powercontrol devices 200 switch between modes, and/or the length of the delayin switching T3, may be varied between different devices. The values maybe assigned randomly (for example within defined limits around definedoptimum values) during manufacture of the reactive power control device200, and stored in the data store 210. In some cases, the processor 206of the reactive power control device 200 may vary one or more of valuesR₁ to R₄ and T3; this could be done according to a randomisation processimplemented at predefined time limits, or on activation of the device byan activation signal from a control centre (see below), for example.Randomising these values in this way prevents distributed groups ofdevices from acting in concert to produce unwanted oscillations in thesystem.

As mentioned above, the reactive power control devices 200 may bearranged to communicate with a control centre via the communicationsinterface. The control centre may be a node in the electricitydistribution network, or any other device arranged to communicate and/orcontrol a distributed group of reactive power control devices asdescribed herein. Communication between it and the reactive powercontrol device 200 could be performed using wireless or fixed linecommunications, for example, communication via the internet and/or a GSMnetwork. In some cases communications between the reactive power controldevice 200 and the control centre may take place by transmission of dataalong the power lines 414.

In some embodiments of the present invention, the reactive power controldevice 200 may be arranged to receive an activation signal from thecontrol centre and control the reactive power provided to theelectricity distribution network 1 in response to receipt of theactivation signal i.e. the activation signal turns the reactive powercontrol device 200 on, with the latter being inactive i.e. turned offprior to receiving the activation signal. When the reactive powercontrol device is inactive, the power device 202 may consume and/orprovide power provided by the electricity distribution network inaccordance with its normal operation i.e. as though it were notconnected to the reactive power control device 200. The control centremay also provide deactivation signals to turn the reactive power controldevice off i.e. into an inactive state.

On receipt of an activation signal from the control centre, each of adistributed group of the reactive power control devices 200 turns on andbegins operating according to, for example, the process described abovein relation to FIG. 6. The generation of random values in relation tothe parameters R₁ to R₄ and/or T3 described above may be performed inresponse to receipt of this activation signal. It may also beadvantageous to arrange the system so that the reactive power controldevices 200 activating in response to the activation signal do not allactivate at the same time, in order to prevent sudden changes inreactive power flowing in the electricity distribution network. This maybe done by arranging each device to activate after a randomly generatedtime interval has elapsed after receiving the activation signal; thisrandomly generated time interval may be generated by the devicesthemselves, or it may be prescribed in the activation signal itself.

In some embodiments, the parameters R₁ to R₄ and/or T3 may be prescribedby the control centre, as part of activation signal, or some othersignal. This enables the characteristics of the reactive power controldevices 202 to be tailored according to the specific conditions of theelectricity distribution network 1.

In some embodiments some of the reactive power control devices 200behave autonomously, without the control of a control centre, and someother reactive power control devices 200 acting under the control of acontrol centre. In this case, it may be advantageous for thepredetermined range described above to be relatively large for theautonomous reactive power control devices 202, and relatively narrow forthe remotely controlled devices 202; in this way, all reactive powercontrol devices 202 react to large swings in reactive power in theelectricity distribution networks, but with only remotely controlleddevices, which may be more finely adjusted by adjusting theircharacteristics remotely, being used to adjust smaller changes.

As described, using a control centre to activate and/or deactivate thereactive power control devices 200 enables the latter to be used todeliver reactive power on demand to the electricity distribution network1. Thus, groups of users of the reactive power control devices 200and/or an operator of the control centre may sell reactive power to thepower provider, for example.

The reactive power control devices 200 may also communicate with thecontrol centre to provide data regarding the performance of the reactivepower control device, and other information. For example, the processor206 may be arranged to record data in the data store 210, such asactivation times of the reactive power control device 200, amounts ofreactive power provided to the electricity distribution network 1,results of measurements of RPC by the reactive power meter 203 etc., andto provide this information to the control centre via the communicationsinterface 208. If the power device 202 is a mobile device, such as aPEV, it may also be arranged to record its location, using e.g. a GPStracking device, and communicate the same to the control centre.

Further, the reactive power control devices 200 may measure one or morecharacteristic of the electricity flowing in the electricitydistribution network 1, and to communicate data indicative of theseelectricity characteristics to the control centre. This measurement maybe performed using the reactive power meter 203, or one or more othermeters. The one or more electricity quality characteristic measured andreported may include one or more of the following:

-   -   A reactive power characteristic;    -   Voltage variations such as dips, sags, swells and brown-outs, in        which the line voltage is higher or lower than the nominal        voltage for a short period of time; this may be caused by e.g.        network faults, switching of capacitive loads and excessive        loading;    -   Harmonics; variations in the line voltage at multiples of the        supply frequency; this may be caused by e.g. power electronic        loads such as variable speed drives and UPS systems;    -   Flicker; random or repetitive variations in the voltage; this        may be caused by e.g. mills, EAF operation (arc furnaces),        welding equipment and shredders;    -   Network unbalance i.e. different line voltages; this may be        caused by single-phase loads, phase to phase loads and        unbalanced three-phase loads like welding equipment;    -   Oscillations (resonances): the flow of electrical energy, e.g.        between the magnetic field of an inductor and the electric field        of a capacitor, changes direction periodically;    -   Transients (fast disturbances): rapid change in the sine wave        that occurs in both voltage and current waveforms; this may be        caused by switching devices, start-and stop of high power        equipment.

The reactive power control devices 202 may also include means to controlthe power consumption and/or provision of the power device 202 toprovide an adjustment to these detected quality characteristics, usingan adaptation of the methods described above in relation to FIGS. 2 to7, for example.

The control centre may use this information for example to determineareas of the electricity distribution network 1 requiring reactive powercompensation. The control centre may then determine a required amount ofreactive power required to be generated by a distributed group of thereactive power control devices 200, and specify a group of the devicesin the relevant network area and send an activation signal to each ofthe specified devices. To this end, it may be convenient for thereactive power control devices 200 to each be individually addressed;for example, each reactive power control device 200 may have an IPaddress and/or each reactive power control device may be fitted with asubscriber identity module SIM card, in which case the address datacomprises an identity number of the SIM card, such as an MSISDN number.

The techniques and methods described herein may be implemented byvarious means. For example, these techniques may be implemented inhardware (one or more devices), firmware (one or more devices), software(one or more modules), or combinations thereof. For a hardwareimplementation, the devices of FIGS. 2 and 4 may be implemented withinone or more application-specific integrated circuits (ASICs), digitalsignal processors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, micro-controllers, microprocessors,other electronic units designed to perform the functions describedherein, or a combination thereof. For firmware or software, theimplementation can be carried out through modules of at least one chipset (e.g., procedures, functions, and so on) that perform the functionsdescribed herein. The software codes may be stored in a data store unitand executed by processors. The data store unit 210 may be implementedwithin the processor or externally to the processor. In the latter caseit can be communicatively coupled to the processor via various means, asis known in the art. Additionally, the components of the systemsdescribed herein may be rearranged and/or complemented by additionalcomponents in order to facilitate the achieving of the various aspects,etc., described with regard thereto, and they are not limited to theprecise configurations set forth in the given figures, as will beappreciated by one skilled in the art.

The above embodiments are to be understood as illustrative examples ofthe invention. Further embodiments of the invention are envisaged. Forexample, in the embodiments discussed above, the electricitydistribution network 1 uses a single phase distribution. However, itwill be clear to the skilled person that the same principles apply tomulti-phase systems, such as three-phase systems.

Further, in the above examples, the current control device 212 isimplemented using one or more semiconductor switching devices. Suchembodiments may be particularly suitable for use with domesticconsumption devices drawing a current of up to approximately 25 to 35amps as is allowed in the art. However, when used with a high powerconsumption or provision device, which produce a level of current whichmay damage the semiconductor switches, it may be preferable to use otherswitching devices, such as vacuum tubes.

It is to be understood that any feature described in relation to any oneembodiment may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the embodiments, or any combination of any other of theembodiments. Furthermore, equivalents and modifications not describedabove may also be employed without departing from the scope of theinvention, which is defined in the accompanying claims.

The invention claimed is:
 1. A reactive power control device for use with a power device, the power device consuming power from, and/or providing power to, an electricity distribution network when connected thereto, wherein electrical power flows in the electricity distribution network, and the power consumed from and/or provided to the electricity distribution network by the power device has different reactive power characteristics as compared to the electrical power flowing in the electricity distribution network, the reactive power control device comprising: detecting means for detecting, at the power device, a first reactive power characteristic, the first reactive power characteristic being a characteristic of electrical power flowing in the electricity distribution network, and relating to a reactive power component of electricity flowing in said electricity distribution network; determining means configured to determine, based on said detected first reactive power characteristic, whether said reactive power component of electricity flowing in said electricity distribution network is more inductive than a predefined value or whether it is more capacitive than a predefined value; and control means configured to control, on the basis of said detected first reactive power characteristic, a second reactive power characteristic, the second reactive power characteristic being a characteristic of the electricial power consumed from and/or provided to the electricity distribution network by the power device, thereby adjusting a value of said detected first reactive power characteristic, wherein the control means is configured to: control said power device to provide a capacitive contribution to the reactive power flowing in the electricity distribution network in response to a determination that the first reactive power component is more inductive than the predefined value; and control said power device to provide an inductive contribution to the reactive power flowing in the electricity distribution network in response to a determination that the first reactive power component is more capacitive than the predefined value.
 2. A reactive power control device according to claim 1, wherein the first reactive power characteristic relates to a phase difference between voltage and current of electricity flowing in the electricity distribution network and the predefined value comprises a predefined phase difference value.
 3. A reactive power control device according to claim 1, wherein: said detecting means is configured to detect a change in said first reactive power characteristic; and said control means is configured to change said second reactive power characteristic, in response to said detecting means detecting a transition of a value of said first reactive power characteristic across a threshold value.
 4. A reactive power control device according to claim 3, wherein said control means is configured to: change a value of said second reactive power characteristic from a first predetermined voice to a second predetermined value in response to a value of said detected first reactive power characteristic changing from a value less than a first threshold value to a value greater than said first threshold value; and change a value of said second reactive power characteristic from said second predetermined value to said first predetermined value in response to a value of said detected first reactive power characteristic changing from a value greater than a second threshold value to a value less than said second threshold value, said second threshold value having a magnitude less than said first threshold value.
 5. A reactive power control device according to claim 3, wherein said control means is configured to change said reactive power in response to a predetermined time period elapsing after said detecting means detects said change in said first reactive power characteristic.
 6. A reactive power control device according to claim 1, wherein the electricity flowing in the electricity distribution network comprises an alternating current electricity flow having a predetermined period, and the control means is configured to control a switching means to selectively interrupt provision of power to and/or from the power device during one or more portions of the predetermined period.
 7. A reactive power control device according to claim 1, wherein the control means comprises a current modulation means.
 8. A reactive power control device according to claim 7, wherein the control means comprises a pulse width modulation means, the power device comprises a power consumption device for consuming power from the electricity distribution network, and the control means is configured to control a duty cycle characteristic of the power consumption of the power consumption device.
 9. A reactive power control device according to claim 1, wherein the power device comprises a power provision device for providing electric power to the electricity distribution network, the power provision device being configured to provide a direct current, and the control means is configured to control a direct current to alternating current conversion means.
 10. A. reactive power control device according to claim 1 comprising means for detecting one or more electricity quality characteristic of electricity flowing in the electricity distribution network, wherein said control means is configured to control a characteristic of power provided and/or consumed by the power device so as to alter said detected electricity quality characteristic, the detected one or more electricity quality characteristic comprising at least one of; harmonics in the electricity flow; random or repetitive variations in voltage; network imbalance; oscillations in power flow; transients in the power flow.
 11. A reactive power control device according to claim 1, wherein the power device comprises a power provision device for providing electric power to the electricity distribution network, the power provision device comprising at least one of: a photovoltaic generation device, a personal electric vehicle, a personal electric bicycle and a domestic renewable energy source.
 12. A reactive, power control device according to claim 1, wherein the electricity distribution network comprises a transmission grid and a distribution grid, the transmission grid being connected to the distribution grid via one or more transformers and providing electric power thereto, and the distribution grid providing power to a plurality of domestic and/or industrial users, wherein the power device is for consuming power from, or providing power to, said distribution grid.
 13. A reactive power control device according to claim 1, further comprising a communications interface for receiving an activation signal from a control center, wherein said control means is a configured to perform said control of said second reactive power characteristic in response to receipt of said activation signal at said communications interface.
 14. A system for use in controlling reactive power in an electricity distribution network, the system comprising: a distributed plurality of reactive power control devices according to claim 13, each controlling a respective power device connected to the electricity distribution network; and a control center for sending a said activation signal to each of the distributed plurality of reactive power control devices.
 15. A system according to claim 14, wherein different ones of the plurality of reactive power control devices are configured to initiate performance of said control at different intervals after receiving said activation signal.
 16. A method for use in controlling reactive power flow in an electricity distribution network connected to one or more power devices, the one or more power devices consuming power from, and/or providing power to, the electricity distribution network, wherein electrical power flows in the electricity distribution network, and the power consumed from and/or provided to the electricity distribution network by the power device has different reactive power characteristics as compared to the electrical power flowing in the electricity distribution network, the method comprising: detecting, at a said power device, a first reactive power characteristic, the first reactive power characteristic being a characteristic of electrical power flowing in the electricity distribution network and relating to a reactive power component of electricity flowing in said electricity distribution network; determining, based on said detected first reactive power characteristic, whether said reactive power component of electricity flowing in said electricity distribution network is more inductive than a predefined value or whether it is more capacitive than a predefined value; and controlling, on the basis of said detected first reactive power characteristic, a second reactive power characteristic, the second reactive power characteristic being a characteristic of the power consumed from and/or provided to the electricity distribution network by the power device, thereby adjusting a value of said detected first reactive power characteristic, wherein the controlling comprises: controlling said power device to provide a capacitive contribution to the reactive power flowing in the electricity distribution network in response to a determination that the first reactive power component is more inductive than the predefined value; and controlling said power device to provide an inductive contribution to the reactive power flowing in the electricity distribution network in response to a determination that the first reactive power component is more capacitive than the predefined value.
 17. A method according to claim 16, comprising: generating a threshold value according to a random value generation process; detecting a change in said first reactive power characteristic; and changing said second reactive power characteristic, in response to a detected transition of a value of said first reactive power characteristic across the threshold value.
 18. A method according to claim 17, comprising: determining a time period value according to a random time period value generation process; and changing said second reactive power characteristic in response to a predetermined time period elapsing after detecting said change in said first reactive power characteristic.
 19. A reactive power control device for use with a power device for consuming power from or providing power to, an electricity distribution network when connected thereto, the reactive power control device comprising: detecting means for detecting, at the power device, a reactive power characteristic, the reactive power characteristic being a characteristic of electrical power flowing in the electricity distribution network and relating to a reactive power component of electricity flowing in said electricity distribution network; determining means configured to determine, based on said detected reactive power characteristic, whether said reactive power component of electricity flowing in said electricity distribution network is more inductive than a predefined value or whether it is more capacitive than the predefined value; and control means configured to: control said power device to provide a capacitive contribution to the reactive power flowing in the electricity distribution network in response to a determination that the first reactive power component is more inductive than a predefined value; and control said power to provide an inductive contribution to the reactive power flowing in the electricity distribution network in response to a determination that the first reactive power component is more capacitive than a predefined value. 